The German Offenlegungsschrift DE 38 26 789 discloses a hydraulic operating system comprising several hydraulic cylinders which serve to move various parts of the foldable roof of a vehicle and are controlled by means of electro-magnetically operated spring-return 3/2-control valves. In the fully opened and the fully closed position of the foldable roof these control valves are not energized, so that the hydraulic system is pressureless.
In order to make it possible to hold the foldable roof in an intermediate position, which position lies between the fully opened and the fully closed position of the foldable roof, the known system provides for a "holding condition", in which a downward movement of parts of the foldable roof under the influence of gravity is prevented. In this position the pump of the hydraulic system is switched off, and a "holding function" is obtained by maintaining energization of the electromagnetic control valves belonging to the working chambers of the cylinders which produce a movement of one or more parts of the foldable roof against the force of gravity. In the case of the known system a non-return valve placed between such a working chamber and the pump prevents hydraulic fluid from flowing out of the working chamber concerned, with the result that the parts of the foldable roof operated by the cylinder concerned are held in place in a stationary position.
The drawback of the known system is that the electromagnetic control valves used for these systems may be energized continuously only for a limited period, because otherwise the electrical part of these control valves can become damaged through overheating. That is why it is proposed in the abovementioned publication that the control unit should be provided with a time switch which stops energization after a specific period has elapsed. The result of this is that the part of the foldable roof which has been "held" by the control valve concerned until then will begin to move under the influence of gravity. This downward movement can be slowed down in a known manner by taking additional measures such as placing a throttle element in the line to the reservoir or by incorporating other decelerating systems, such as friction couplings, in the foldable roof mechanism. In the case of some roofs of stiff fabric material the downward movement is slowed down to some extent by the resistance of the fabric to folding. However, in the case of foldable roofs with relatively heavy and rigid roof parts in particular, the downward movement will be very rapid unless it is deliberately slowed down. This can lead to injury to the passengers in the vehicle, or in any case to a shock reaction.